1. Field of the Invention
The present invention relates to an apparatus and method for supplying a release coating to a fixing roller or similar device, such as those commonly found in various printer devices.
2. Description of Related Art
Fuser technology is employed today in a wide variety of printer devices, such as plain paper copiers and fax machines, laser printers, etc. In these devices an image is formed by toner, typically a blend of thermal plastic, wax, metal oxide, and/or carbon, fixed to paper by passing it through a nip between a heated fixation roller and a pressure roller (herein sometimes referred to interchangeably or collectively as "fuser roller"). As the paper passes through the nip, toner facing the hot fixation roller melts and flows into the paper. This area of copiers and printers is typically referred to as the "fuser."
In order to prevent the toner from sticking to the fixation roller during fusing of the image, a release agent is typically applied to the fixation roller. Silicone oil (or dimethylsiloxane) is the release agent of choice in most copier and printer applications. However, amine, or mercapto functionalized silicone fluids, as well as hydrocarbons, natural oils, and water may be used as "release agents." The release agent is transferred to the paper during fusing and promotes the flow of the toner into the paper. When there is an inadequate amount of release agent on the fixation roller, the toner will become adhered to the fixation roller during the fusing process and can become deposited on subsequent pages or print, creating undesirable spots, which is referred to in the industry as "offsetting."
The trend in the non-impact printing industry is to produce images with higher resolution. This means that there are more dots per inch (DPI) on prints and copies. In order to achieve this finer resolution, the toner particle size must be smaller and this has led to some problems. With finer resolution particles, the standard amount of release agent is no longer acceptable and will in fact lead to pick up of smaller toner particles during the fusing process. It is therefore very important for good print quality that there be a substantial, consistent, and even layer of release agent on the fixation roller.
The release agent delivery device for current non-impact printers has to supply the appropriate amount of release agent consistently over the life of the part, and must be able to collect and hold any paper dust or offset toner. These two functions are critical to the proper functioning of the printer or copier. Many existing release agent delivery devices can usually provide one or the other function effectively, but all have deficiencies.
Aramid fiber (e.g., NOMEX.RTM.) release agent delivery devices have been used extensively in printers for many years. The devices come in a variety of geometries suited for the needs of various printer machines, including non-woven webs, and woven or felted stationary wicks. Unfortunately, NOMEX.RTM.--type fibers are coarse and do not have the ability to adequately control the rate of oil delivery. In many of the applications, the NOMEX.RTM. fibrous material is saturated with silicone oil and then pressed against the fixation roller. These devices deliver an inconsistent amount of oil and can be very abrasive on the fixation roller surface. In addition, NOMEX.RTM. fiber web materials come in many different forms, all of which have extremely high variations in density and thickness. These variations cause oiling irregularities and fluctuations that cannot be tolerated. Other problems with these forms of webs and stationary wicks include:
1) Decreasing oil delivery over the life as the oil drains out;
2) Oil leaking out in null periods, leading to high initial oil rates;
3) Pores clogging with dirt over time, which will adversely affect the oil delivery;
4) Building up of static electric charges when electrically insulative material is used;
5) Premature wearing of the fuser roller due to abrasive surface and high contact pressures;
6) Poor efficiency of oil transfer;
7) May require additional oil delivery apparatus, such as pumps or reservoirs; and
8) Settling or "puddling" of oil in the lower hemisphere of the roll upon null periods which leads to one half of a circumference length of low oil web and the other half of high oil, which may lead to poor image quality.
Other stationary oil delivery devices attempt to improve the oil delivery rate and reduce the abrasion of the NOMEX.RTM. by covering the NOMEX.RTM. felt with a protective cover, such as an expanded polytetrafluoroethylene (ePTFE) membrane. These devices have limitations in operating life and demonstrate significant inconsistencies in oil delivery over the operating life.
Significant improvement in performance has been achieved by applicants in stationary oiler designs by mounting oiling media into a tube of expanded polytetrafluoroethylene (PTFE). Such devices are described in U.S. Pat. No. 5,478,423, which issued on Jan. 26, 1995, and copending U.S. patent application Ser. No. 08/127,670.
Still another approach is to employ a rotational oiler device. One example is described in U.S. Pat. No. 5,232,499 to Kato et al. This approach solves some of the problems listed above but does not provide all of the needed characteristics. The oiler rotates against the fuser which eliminates most of the wear problems on the fuser, but this does not facilitate collection of offset toner and paper dust. Further, the oiler delivers the oil through diffusion, so the rates of delivery can be limited to very low amounts. Finally, the oiler still utilizes a reservoir which diminishes over the life of the part and still can lead to inconsistent oil delivery rates.
Oiling webs are a simple and effective way of addressing many of the problems discussed above. A web has oil self contained within it and therefore will deliver the oil consistently as the web is indexed to expose an unused portion of the web in contact with the fuser roller. In addition, the web has all of the oil contained within the pores of the material and therefore does not require a separate reservoir of oil which, depending on the configuration of the assembly, can be messy and difficult to meter. Webs tend to have superior cleaning ability because the collected toner and dirt is removed from the fuser roller with the taken up web material.
There have been a number of attempts to make an oil delivery web for copiers and printers that can meet all of the needed requirements. To date, however, all of the attempts have had shortcomings in one area or another.
One deficient approach to web design has been a composite web of aramid and thermoplastic blend nonwoven fabric. This web material has proven to be abrasive and to cause premature wearing of the fuser roller, which is typically coated with either silicone rubber or fluoropolymer. Further, aramid and thermoplastic web material can only hold a very small fraction of oil in its matrix. Typical oil holding capacities are approximately 30 to 50%. Also, the material has limited control of oil delivery due to the relatively inconsistent and overly large void spaces within the material. The material often has large variations in density and thickness (i.e., about 10% or more in both). Further, the material has high in-plane oiling, which results in inconsistent oil delivery rates and less than complete oil delivery. Typically, an aramid web delivers only about half of the oil contained within it (which starts off at only about 30-50% of the volume of the material). This is a waste of oil and requires more web to be used for a given life expectancy of oil delivery.
Another web material described in PCT/GB92/01958 utilizes a porous polytetrafluoroethylene. This material is a non-expanded PTFE material and comprises particles of PTFE that are sintered together to form a coherent matrix of particles and voids. This isotropic material has relatively large pore sizes and exhibits homogeneous wicking properties in the through direction and the plane direction. This homogeneity limits the control of the oil delivery and prevents the material from having complete oil delivery. Additionally, the larger pore size means that low viscosity oil will not be retained within the pores. In some applications, extremely thin oils are required, down to approximately 50 cst, which is too thin to be held within this material.
Another problem is that sintered PTFE material such as that disclosed in PCT/GB92/01958 is brittle and, thus, has to be relatively thick to avoid breakage in use. Where space constraints are a problem, the necessary thickness of the material means that less material can be used due to space constraints. Typically, the material is 0.010" (0.25 mm) thick or thicker in order to provide enough structural integrity for a web application. This material is suitable for some applications, but in no way addresses all of the demands for printer applications, especially those applications in which a lot of release agent is necessary. Furthermore, the sintered PTFE particle material has extremely low elongation, which causes it to prematurely crack, break or tear in applications if the stress applied is too high.
Accordingly, it is a primary purpose of the present invention to provide an apparatus for applying release chemicals to a roller, belt, or mating surface which is durable, delivers a consistent coating of chemical to the fuser, and provides effective cleaning of the fuser roller and high efficiency (oil transfer).
These and other purposes of the present invention will become apparent by the following specification.